1. Field of the Invention
The present invention relates to a circuit for the production of a transmission level indicator signal during transmission of signals which have to be interpreted in dependence of their exceeding a prescribed transmission level in tele-communication facilities and especially data-transmitting systems.
2. Description of the Prior Art
In the transmission of signals, for example via MODEMS, base-band units and the like, error-free interpretation must be achieved at the receiving station. For example, errors can be caused on the transmission channel and even error-free receiving stations can produce wrong interpretation results. Moreover, such errors typically adversely effect the level of the received signals. Thus, if the signal level does not reach a prescribed minimum value at the receiving station, it is attributed to a disturbance and/or transmission error. For this reason, signal-transmission systems produce a so-called transmission-level-indicator-signal indicative that the received signals have a level which permits secure interpretation of the signals. This guarantees that, on one hand, only signals of a sufficient level are interpreted, while on the other hand interpretation of disturbance signals below a prescribed level is prevented.
During signal transmission, a fluctuation in the level of the transmission signals as well as disturbances and/or humming in the transmission channel can occur. Such fluctuations are sometimes so strong that at a steady ratio of signal : noise, the prescribed level of transmission can be undercut from time to time. This can lead to a blockage of signal interpretation even though the magnitude of signal : noise is sufficiently large. Moreover, certain modes of transmission require numerous switch-on-switch-off operations at the receiving (interpretation) station which, due to switching and interpretation delays also block signal interpretation although signals of sufficient magnitude are present. A transmission type of this kind is, for example, the so-called half-duplex-operation in which between two participating points of data exchange, a signal is sent from one station to the other and the other station returns a confirmation signal. In order to maintain the time necessary for switching from "send" to "receive" as minimal as possible and utilize the time for actual signal transmission most effectively, it is especially desirable to utilize a transmission level indicator signal which is independent of the switching process and free of delays.